1. Field of the Invention
The invention relates in general to a hole metal-filling method, and more particularly, to a hole filling and electroplating method for a printed circuit board which has been mechanically drilled with holes.
2. Description of the Related Art
In addition to the great demand in functionality, the consumers also require the electronic product to be light, thin, short and small. Therefore, the integration has to be higher and higher with more and more powerful functions. Consequently, the printed circuit board (PCB) to equip the electronic devices is fabricated with more and more layers, for example, from a single layer to 2, 6, 8 or even 10 layers. The electronic devices can thus be formed on the printed circuit board more densely with a smaller occupancy of surface area.
However, as layers of the multi-layer circuit board increase, the fabrication process becomes more complex, and the production time is also lengthened. The subsequent testing process is also increased with more complex steps and longer time.
In FIG. 1A, an inner substrate 14 with two layers of circuit 12 is shown. The circuit 12 can be located at two sides of the insulation layer 10 and formed using photolithography and etching process.
In FIG. 1B, a surface of the inner circuit 12 is oxidized to obtain a coarse surface, so as to improve the bonding performance between the inner circuit 12 and the insulator. A layer of rubber sheet 16 and a layer of copper foil 18 are located and laminated at two sides of the substrate 14. The next layer of circuit is then fabricated at two sides of the substrate 14. However, the fabrication of the next layer of circuit has to be performed after bonding and curing the rubber sheet 16 with the inner circuit 12. To bond the rubber sheet 16 with the inner circuit 12, the rubber sheet 16 has to be heated and pressed with a process called thermal press. The rubber sheet 16 can then be hardened and closely adjacent to the inner circuit 12. The temperature is then reduced, while the press is continuously applied in a cold pressing process.
In FIG. 1C, the interconnecting holes and the component holes 20 to install components and to interconnect various layers have to be drilled first. The holes 20 are then plated with copper with a plating through hole (PTH) process to form a copper foil in the holes. An outer circuit layer is then formed using photoresist layer to etch the copper foils 18, 22.
Generally speaking, an electroplating process is used to form the copper foil 22 in the hole 20. To plate a cylindrical shape of copper in the hole 20, a very long plating time is required. Thus, the cost is high, and the applicability of the hole 20 with a diameter more than 0.2 mm is not desired.
The invention provides a metal-filling method of a hole. After forming and electroplating a hole in a printed circuit board, this method is performed. In this method, a substrate is provided. A plurality of holes is drilled through the substrate. The substrate is placed on a platform, and a plurality of metal balls is disposed on the substrate. By vibrating the platform, a part of the metal balls roll into the holes. The metal balls not rolling into the holes after vibrating the platform are removed. The substrate is then placed on a press down unit and pressed until the metal balls within the holes are level with the surface of the substrate. An electroplating step is performed on the substrate directly to form a plating layer closely adjacent to the metal balls.
While applying the above method to printed circuit board, as copper is selected as the material of the metal balls, the diameter of the holes in the substrate can be increased. Due to the good electrical and thermal conductivity, the copper metal balls can also used as heating dissipation members such as ball grid array (BGA) package device. In addition, while replacing laser via with mechanical via in the product with high density interconnection (HDI), the fabrication cost is greatly reduced.
The laser used typically includes gas laser, solid laser such as CO2 laser, yttrium-aluminum-garnet (YAG) laser with a wavelength of 10.6 micron, 1.064 micron and a beam size of 0.1 mm and 0.05 mm, respectively.
Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.